Electronic transducer



Dec. 13, 1949 H. F. OLSON ELECTRONIC TRANSDUCER Filed Oct. 51, 1946 2 She ets-Sheet 1 IIIQl/E'AC/ f,

fife/ 12 0/1022, By I Gttorneg Dec. 13, 1949 F, OLSON 2,491,390

' ELECTRONIC TRANSDUCER Filed Oct. 51, 1946 2 Sheets-Sheet 2 ilyjj'.

Z'suventor Patented Dec. 13, 1949 ELECTRONIC TRANSDUCER Harry F. Olson, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application October 31, 1946, Serial No. 706,967

12 Claims.

This invention relates to electronic transducers, and more particularly to a transducer suitable for reproduction of sound from a phonograph record.

Many types of electrical pick-up devices for use with phonograph records have been suggested heretofore, among them electronic devices employing an evacuated tube or shell having two or more electrodes therein one of which, such as the anode or the grid, is arranged to be movable relative to the others and has coupled thereto a phonograph needle. When the needle is actuated by the signal groove of a record, it actuates the coupling means between it and the aforesaid movable electrode to move the latter and thereby produce in the output circuit of the tube variations in current corresponding to the recorded signals. In spite of a number of advantages afforded by pickup devices of this sort, one very important reason Why such pick-up devices have not been put into commercial use is that difficulty is encountered in transferring the vibrations through a vacuum tight shell such as is necessary in devices of this sort. Within the tube or shell, there is a vacuum, while on the outside there is ambient air pressure. The differential in pressure is therefore approximately fifteen pounds per square inch. Now, the actuating forces encountered in the reproduction of sound from phonograph records are of the order of a millionth of a pound. To provide a vacuum tight link between the needle and the movable electrode within the tube or shell that can be actuated by forces of such small magnitude and that will withstand fifteen pounds per square inch of static pressure is a problem which long confronted the art without successful solution to a point where a commercial electronic pick-up device has been feasible.

Aside from the foregoing, my investigations have shown that consideration must be given to the vibrating system of such a transducer if it is to be operated at suitable efficiency. The volt age developed by the electronic generating system of the type under consideration is proportional to the amplitude of vibration of the vibratile electrode. Therefore, to maintain constant voltage output over a certain frequency band, the amplitude of vibration must be independent of the frequency over that frequency range. Under these conditions, the velocity is proportional to the frequency and if the system is mass controlled the mechanomotive force will be proportional to the square of the frequency. Due to these factors, namely, the constant amplitude requirement and the square of the frequency function for the mechanomotive force, it is evident that the mechanical mass reactance will govern the ultimate sensitivity of any system in which the available driving force is limited or fixed. For this reason, it is; extremely important that the effective mass 2 of the driving link in the electronic transducer be made as small as possible.

The primary object of my present invention, then, is to provide an improved electronic transducer of the type under consideration in which the driving link or rod between the source of the driving force and the movable or vibratile electrode will have a small mechanical mass reactance.

More particularly, it is an object of my present invention to provide an improved electronic transducer which is suitable for use in phonograph pick-ups, microphones, and other similar devices, and which will operate with maximum efficiency.

Another object of my present invention is to provide an improved electronic transducer which can be replaced easily without requiring the service of one with technical training.

A further object of my-present invention is to provide an improved electronic transducer which will not be affected by temperature or humidity changes.

Still another object of my present invention is to provide an improved electronic transducer as above set forth which will have an electrical impedance that renders it particularly suitable for use as an electric pick-up device for phonographs in contrast to the relatively high impedance which characterizes so-called crystal pick-ups and the low impedance of so-called magnetic pick-ups from the standpoints of transmission and electrical noise pick-up.

It is also an object of my present invention to provide an improved electrical transducer as aforesaid which can be manufactured easily, which will be practically free from trouble in use, and which will be very efficient for the purposes to which it may be applied.

In accordance with my present invention, the transducer consists, essentially, of an evacuated vessel, such as a metal tube envelope, one wall of which, such as the base, is constituted by a thin, metallic diaphragm. The tube may be a diode, in which case the plate or anode is arranged to be the movable electrode, or it may be a triode, a pentode or any other suitable type of multi-electrode tube, in which case the grid may constitute the movable electrode. In any case, the movable electrode is mounted on the aforesaid diaphragm which constitutes a flexible support therefor. Also coupled to the diaphragm is an actuating link which, in the case of a phonograph pick-up, may carry or be coupled to a phonograph needle, or in the case of a microphone, may be coupled to a suitable vibratory diaphragm adapted to be set in vibration by sound waves impinging thereon.

The movable electrode, whether the anode of a diode or the grid of a triode or the like, will be referred to hereinafter as the control electrode. This control electrode should preferably extend in a direction generally parallel to that of the cathode or electron emitting electrode. The control electrode may be of various cross sections'as will appear more fully hereinafter, but I have found from my investigations that a control electrode fundamental vibrating system for transferring which is tapered, and preferably conical, with its a base end secured to the diaphragm wall of the envelope or shell provides best results.

The novel features of my invention, as well as additional objects and advantages thereof, will 7' better be understood from the following description of several embodiments thereof, when read in connection with the accompanying drawings in which 7 Figure 1 is a view partly in section and partly diagrammatic showingi generally, one form of transducer in accordance with my present invention,

Figure 2 is a wiring diagram showing the elec-' trical analogue of the mechanical network of Figure 1,

Figure 3- is a curve showing the response fre quency characteristic of the form of my invention shown in Figure 1 when the vibratory bar or rod is stiff and its mechanical resistance is small, the solid line curve showing the characteristic without damping, and the dotted line curve showing thezcharacteristic with damping,

. Figure 4 is a similar curve in which all the elements of the vibrating system are effective in the'response range, the mechanical resistances of the various parts of the system being small,

Figure 5 is a side elevation, partly in section of one form of pick-up device for use with phonograph records in accordance with my present invention;

Figure 6 is a sectional view of one form of microphone in accordance with my present invention,

. Figures 7cr-I2a,inelusive, are graphic viewsv of cantilever bars of various configurations showing the various forms: in which the control electrode of mypresent invention may be made,

, Figures lb-+121) are end views thereof as seen from the right of each of Figures la-12a,

Figure 13 is a graph showing the. deflection of a conical and a cylindrical control electrode such as illustrated in Figures 7a,. 'lb, and. 12a, 1212,

Figures 14 and 16 are side elevations, partly in section, of other forms of phonograph pick-up devices according to my present invention,

. Figure 15 is a. wiring diagram showing the electrical analogue of the vibratory systems of Figures 14 and 16, I,

Figure 17 is a sectional view of another form of microphone in accordance. with my present. invention, and

Figure 18 1s awiring diagram showing the electrical analogue. of the mechanical network of the vibratory system of the microphone of Figure 1'7.

Referring more particularly to the drawings wherein similar reference characters indicate corresponding parts throughout, there is shown, in Figure 1', an evacuated. vessel comprising a metallic shell! having an opening in the wall structure which is closed by'a thin, flexible, metallic diaphragm. 3. Within thev shell or vessel t is a cathode 5 which may be heated by a filament 1,. and. an anode 9a. which is constituted by the inner portion of an elongated rod or bar 9 which extends through and is secured toa flexible diaphragm 3, the rod 9 having a portion 9b which is external to thevessell. M is apparent, therefore, that since the bar or rod 9-is anchored at the diaphragm 3,, its inner portion (that 15,. thew;- wares impinging thereon.

vibrations through the vacuum tight shell I. The mechanical network of the vibrating system of Figural. is shown iii Figure 2 wherein F represents the driving force applied at the outer end of the rod portion 9b; mi, 11, and 01 represent, respectively, the effective mass, the mechanical resistance and the compliance of the rod portion 911; m2, 7'2, and 02, respectively represent the effective mass, the mechanical resistance and the compliance of the anode or internal portion' a of the rod 9; and ma, rs, and 03', respectively represent the effective mass, the mechanical resistance and the compliance of the diaphragm 3. The damping in the metallic system of Figure 1 is practically negligible and for this reason the response will be extremely high at the resonant frequencies of the coupled system. By suitable selection of constants, either or both of the resonant freouencies of the rod portions 911 and 91) may be located inside or outside of a prescribed frequency band.

The response frequency characteristic of an electronic transducer such as shown in Figure 1 and in which the bar 9 has a very high stiffness is shown by the solid line curve of Figure 3. In. thisv case, the effect of the compliances c1 and cs. are negligible in the frequency range under consideration. For this condition, there is only one resonant frequency at the frequency ii. The ratio of the amplitude to the force is independent of the frequency below the resonant frequency. A transducer of the type shown in Figure 1 in which all the elements of thevibrating system are effective in the selected response range is shown by the. solid line curve of Figure 4. Here, it will be noted that there are two peaks at the frequencies f2 andtfr. The sharp peaks in the response range are undesirable and must be damped in transducers used in the reproduction of sound. The mechanical system outside the shell I may be damped, if desired, by the application of damping material to the rod portion 92) and also to the diaphragm 3, as will be described more fully hereinafter with. reference to Figure 5. If desired, also. the rod 91 may be made of hollow tubing with damping material within it. The response frequency characteristics will appear as shown by the dotted line curves of Figures 3 and 4 when damping is applied. In the latter case, .the. mechanical resistances are relatively large.

In Figure 5, there is shown a pick-up device utilizing the features of my invention discussed above. Here, the metallic shell or vessel I is secured to a pick-arm arm I! and the rod portion to is provided at its free, outer end with a needle [3 for cooperation with a phonograph record I5. A block of damping material ll, such as rubber or Viscalo'id', is interposed between the rod portion 96 and the arm H, and the strip or pad of similar damping material 9 is applied to the diaphragm 3'.

In Figure 6-, there is shown a microphone utilizing an electronic transducer as above described. This microphone has a casing 21 within which the shell or vessel 1 is supported by means of a supporting block 23. The casing 2| has an opening 25: therein; over which is mounted an acoustic diaphragm 21"for vibration in response to sound The diaphragm 21.

may be coupled to the free end of the rod Portion 9b by means of a compliant link 29.

In all of the above modifications, the rod or bar 9, including its anode portion 9a, is of cylindrical form. In wide range transducers, it is desirable to make the effective mass reactance of the anode portion 9a as small as possible. This can be accomplished in accordance with my present invention by utilizing bars or rods of tapered cross section since such bars will exhibit a higher resonant frequency for the same effective mass, or a smaller effective mass for the same resonant frequency.

As will be obvious from an inspection of Figures 1, 5 and 6, the anode portion 9a of the bar 9 constitutes a cantilever bar with respect to the diaphragm 3 on which it is supported. The fundamental resonant frequency, in cycles er second, of a cantilever bar of uniform, circular cross section as shown in Figures 7a and 7b (that is, a cylinder) is given by the equation .56 W f 13 12 where b=thickness of the bar in the direction of vibration, in centimeters.

The resonant frequency of a wedge shaped bar vibrating in a direction normal to the two parallel sides of the wedge, as in Figures 9a and 9b, is given by the equation f= if where b=thickness of the bar in the direction of vibration, in centimeters.

The resonant frequency of a wedge shaped bar vibrating in a direction parallel to the two parallel sides of the wedge, as illustrated in Figures 10a and 10b, is given by the equation The resonant frequency of a pyramidal bar with a square base vibrating in a direction parallel to the base, as shown in Figures 110. and 11b,

is given by the equation 1.20 Q2 f 12p where b=length of the base along any side thereof, in

centimeters. The resonant frequency of a conical bar, such as shown in Figures 12a and 12b, is given by the equation [E f 1' 4p where a=radius of the cone at the base, in centimeters.

The resonant frequency of a hollow, cylindrical pipe (not illustrated) is given by the equation f J (7) where a=outside radius of the pipe, in centimeters, and a1=inside radius of the pipe, in centimeters.

A consideration of the above equations shows that, for a certain resonant frequency, the conical bar of Figures 12a and 1222 will yield the lowest effective mass. If the fundamental resonant frequency of the cylindrical bar of Figures 7a and 7b is made the same as the conical bar of Figures 12a and 12b, the ratio of their radii for the same length will be Radius of cylindrical bar Radius at base of conical bar The ratio of the total masses of the two bars under these conditions will be T0ta1 mass of cylindrical bar (2.5)

10- Total mass of conical bar .33

The mass referred to the free, inner end of the anode portion 9a of the bar is termed the effective mass. In the case of the electronic transducer, when the compliance of the diaphragm 3 is very large compared to the compliance of the rod 9, and in the frequency below the resonant frequency of the rod 9, the rod moves as a rigid member with the diaphragm 3 as a fulcrum. The effective mass of the cylindrical bar under these conditions is given by the equation 2KE ME: 5-1 (10) where I E=kinetic energy stored in the bar, and 6l=velocity at the end of the bar.

The effective mass may be expressed as 1 2 m =J mydib (11) 1 1 Zm m (12) where m1=mass per unit length, and mu=total mass of the bar 9a.

The effective mass of a cylindrical bar 9a with the base as a fulcrum is the total mass of the bar.

The effective mass of a conical bar with the base as a fulcrum is The effective mass of a conical bar with the base as a fulcrum is 1/10 the total mass of the bar.

. EForrthesamez'fzundamentaliresonant frequency, theratioofatheaeffective masses,rfromEquationsQ, 12 and 16, wilLbe "This means'that belowthe' resonant'irequency, where "the *vibrating system is "governed by the mass, the mechanical reactance for the conical L'bar "Will-ibe lthat-pf the eoylindrical lbar. 'In the-case of a systemin which the mass of the vibrating b-aris the controlling mechanical reactance, the sensitivity-will be'increased approximately 36 db. by the useof thesconical bar. In'the case of the aphonograph pick-up, the mechanical reactance presentedtothewecord@may be reduced by the factor of 63 to 1. I I

The deflection of a cylindrical bar at resonance withereierence to' itsneutral or static .position: is .given by'the equation -x=distance along %'the barrportionta from the .cfdiaphragmw.

,flhedeflection Ofafi conical .bar at resonance is ive .by

'5ac=6l%; 1

"The deflection curvesfor cylindrical and conical bars are shown in 'Fig. 13. It is interesting to note, in passing, that the difference between the deflection characteristics. is :verysmall. .At resonance, the effectivemass of thecylindrical bar is =-At resonanceythe effective mass of a conical bar is For the same resonant frequency, the ratio of the effective masses from Equations 9, 21 and 23 1s The relative effective masses at resonance are in the ratio of 166:1. A small effective massat resonance is extremely-important not only-from the standpoint of sensitivitybut from the standpoint of damping as well. The mechanical resistance required for the same response characteristic is also in the ratio of 166:1. This means that the problem of damping is tremendously simplified in the case of the conical bar.

"In the preceding analysis, it has been shown that the effective mass of a conical bar is /s3 the effective mass of a-"cylin'drical bar below a the -resonant :='frequency--of the "bar and that, at the tion Sawithinthe shell I.

resonant frequency, theeeffective mass of the conical bar is V the effective massofa cylindricalbar. The advantage of makingthe rod portion 9 conicalwill be readily-apparent. Other tapered bars, such as those of Figures 9a, 10aand 11a also'offer a marked advantage over the cylindrical bar of Figure 7a and the rectangular bar of Figure 8a. However, the conical bar or rod of Figure 12a provides best results.

In Figure 14, I "have shown .anotherforrnof phonograph pick-up device according to my present invention and in whichthe anode portion 9a of the bar which'issupported'by the diaphragm 3 is in the form ofa cone. Preferably, also, the external-portion "9b of thebarfiis in=the form of a cone which tapers oppositely to thBaIlOdfilPOr- The outer or tip portion of the bar section-9b carries a socket 30 in which the phonographneedle I3 is mounted. The

may be of the typesshown, for example, in the Hasbrouck Patent No. 2,326,460 and which carries the phonographneedle H3 at itsouter end. This modification of "my invention j permits arranging the envelope l of the transducer horizontally (that is, with theaxisof the diaphragm .3 horizontally) and this inturn, permits a reduction 'in the vertical dimensionsof the pick-up'arm II.

The mechanical network of the arrangements of Figures 14 and 16=is illustrated in the wiring diagram of Figure 15 whereF is the force applied at the needle l3; Z represents the mechanical impedance of ,,the,record I5; .1111 represents the mass ofneedle I 3'an'd'its'socket 30, the rod portion 91) andlin the case of Figure 16, alsothe wire element T1 and 01 represent, respectively, the mechanical resistance'and the compliance of the portion ofthe vibrating system external to the shell I man and 02 represent, respectively, the mass, the mechanical resistance and the compliance of the rod portion or anode 911; m3,

rs and 03 represent, respectively, the mass, the mechanical resistance and the compliance of the diaphragm -3; and 1124 and 'rrrepresent, respectively, the mass'ofthe pick-up arm I land the resistance at the pivot 3 I.

In Figure 17, there isashown another form of microphone employing-atransducer in accordance with my present invention. The microphone of Figure 1'7 is generally similar to that shown in Figure .6 except thatathe'case" 2 I is here provided with a second opening 31 in its front wall adjacent the opening 25 and with an inwardly-directed tube 39 leading from the-opening '31. The opening 31 and tube 39 provide a small port or vent which introduces an additional degree of freedom and makes it possible "to accentuate the response at the low frequency portion of the selected frequency range, and at the same time to obtain a sharp,;low:frequency--cut-ofl.

Figure 18 shows an electric wiring diagram which is the analogue of the mechanical system of the microphone of Figure 17. The two driving forces F1 and F2 are equal and opposite in phase.

The driving force F1 is given by the equation F1=p1AD (25) where An=the area of the diaphragm 21, and pi=the sound pressure at the diaphragm 21.

The driving force F2 is given by the equation F2=p2AD (26) where An=the area of the diaphragm 21, and p2=the sound pressure at the port 31, 39.

of the air in the port 31, 39; 04 represents the compliance of the air within the casing 2|; ms, 7'5 and represent, respectively, the mass, the mechanical resistance and the compliance of the diaphragm 21; and re and 66 represent, respectively, the mechanical resistance and the complianc of the link 29.

At the high frequencies, the mechanical reactance due to the compliance or is small compared to the mechanical impedance of the port 37, 39 (that is, mr, r4). Under these conditions, the system is driven by the force F1. At the extreme low frequencies, the mechanical reactance of the compliance or is large compared to the mechanical impedance of the port 31, 39 (again, m4, T4) and since F1 and F2 are of opposite phase, the net driving force is practically zero. In the region where the mechanical reactance due to the compliance c4 and the mechanical reactance due to the port 31, 39 (or 1124, m) are comparable, the addition of this network introduces a phase shift of such magnitude that both F1 and F2 contribute in driving the mechanical system.

An electronic microphone of this type is an exitremely simple device. It consists of the electronic transducer, a sound wave responsive diahragm, and the case. For this reason, the cost should be very low. A microphone of this type would be suitable for sound reinforcing systems, public address systems, call and paging systems, outside broadcast pick-up, amateur radio, home recording, and many other uses. Because of its low cost and its high output, an electronic microphone Of this sort is advantageous for general sound applications, particularly since a microphone of this sort can be made to have the same sensitivity as the carbon microphone but without the disadvantages of high distortion, carbon packing and variation of response with orientation inherent in the carbon microphone, while at the same time possessing the good articulation characteristic of dynamic or magnetic microphones.

Although I have shown and described several embodiments and applications of my present invention, it will undoubtedly be apparent to those skilled in the art that many other forms thereof and applications therefor are possible. I therefore desire that the foregoing description shall be taken merely as illustrative and not as limiting.

I claim as my invention:

1. An electronic transducer comprising an evacuated vessel having a flexible diaphragm as part of its wall structure, and a plurality of electrodes therein one of which is fixed and is adapted to emit electrons and another of which joins and is mounted on said diaphragm and extends therefrom in a direction generally parallel to said one electrode in spaced relation thereto, said other electrode being of tapered configuration in the direction of extension from the diaphragm and being movable toward and away from said one electrode upon flexure of said diaphragm.

2. An electronic transducer comprising an evacuated vessel having a flexible diaphragm as part of its wall structure, and a plurality of elec trodes therein one of which is adapted to emit electrons and another of which is of tapered configuration .and is mounted on said diaphragm at its largest cross section, said other electrode extending in a direction generally parallel to that of said one electrode in spaced relation thereto and being movable toward and away from said one electrode upon flexure of said diaphragm.

3. An electronic transducer comprising an evacuated vessel having a flexible diaphragm as part of its wall structure, and a plurality of electrodes therein one of which is adapted to emit electrons and another of which is mounted on said diaphragm for movement toward and away from said one electrode upon flexure of said diaphragm to thereby control the flow of electrons toward itself, said other electrode comprising a conical member extending in a direction generally parallel to that of said one electrode in spaced relation thereto.

4. An electronic transducer comprising an evacuated vessel having a flexible diaphragm as part of its wall structure, and a plurality of electrodes therein one of which is adapted to emit electrons and another of which is mounted on said diaphragm for movement toward and away from said one electrode upon flexure of said diaphragm to thereby control the flow of electrons toward itself, said other electrode comprising a conical member extending in a direction generally parallel to that of said one electrode in spaced relation thereto and being connected to said diaphragm at its base.

5. An electronic transducer according to claim 2 characterized in that said other electrode is of gradually converging taper in a direction away from said diaphragm.

6. An electronic transducer according to claim 2 characterized in that said other electrode comprises a wedge-shaped member.

7. An electronic transducer according to claim 2 characterized in that said other electrode comprises a pyramid.

8. An electronic transducer according to claim 2 characterized in that said other electrode extends through said diaphragm, and characterized by the addition of means coupled to the portion of said other electrode which is external to said vessel for imparting vibrations to said other electrode upon application of forces to said means.

9. An electronic transducer comprising an evacuated vessel having a flexible diaphragm as part of its wall structure, a first electrode therein adapted to emit electrons, at second electrode therein of tapered configuration secured at its base to said diaphragm and extending inwardly z ceaawm I i? from: said diaphragmz generally in? as diteetiom parallel to said1first electrode: inzspaced 'relationt thereto; said-18906115 iele'ctro'de being movableltv ward and away from said-first ele'ctrode mpon flexure of said I diaphragm to tl-ier'eby contm'l the 110w of electrons tcward itself; and 1 means connee-tedito said diaphragm external td-sa'id vessel 1 for effecting flexure"thereof whereby tti move said second electrode relative to said first electrode? 1'0.-An electronic transducer according" to"- cla-iin- 9 wherein said means inclizd'e'sphonerapri needleadaptedtocooperate with'aspliono graph: record;

11 An electronictransducer according to" claim 9- wherein said means meme-es asecond? and vibratory' diaphragm adapted" to' lias'etinto vibration by= acoustical waves impinging? thereon.

12$ low mass; light* weight transducer unit for u's'e in electronic phonogTaph-piclupsand the? like; comprising-a relatively small "electronidtiibe? 0.

shell having an opening in the-- wall structure thereof; a thin flexible diaphragm-providing-c10 sure" means 'sealing'jsaidopening; a: fixed cathode element-'in'said'she'll; a tapered'cont'ro'l electrode fiXedatdt'sbase-tb 'saiddiaphragmand "extending 2? inwardly: therefrom: 1m spaced: relatien to: said catne'de element? sald' control electrode having-am extenswm element secured to said diaphragm ex;- te'r'n'allyfofz'said-lsh'ell for imparting movement toa, said control el ectrodei with respect to; the fused electrode in response to an applied force of.. a magnitude to flex said diaphragm, said control electrode and extension thereof having a rela'-- tively low; momentofinertia 1 about the: connect tion with the diaphragm: and, at relatively small REFERENCES GIT-ED fol-lowing; references-2* aver of: record?- im the files of: patent-3 UNITED STATES? PATENTS Number Name Date:

2, 1425857" McArthur; J an; 3:, 1939 2,165,981; Sampson: v lJulyj 11,. 193.9? 

